How do email clients know / identify meeting invitations (when amending or deleting)?

Question

I'm rather curious to understand how Outlook and/or Google Calendar know which calendar entry to either amend and/or delete, once the original has been accepted?

Here's a quick scenario:

1. I receive a meeting invitation for tomorrow @ 2pm
2. I accept the meeting invitation
3. I receive a meeting modification for tomorrow @ 3pm
4. I accept the modified meeting

But the question is how does the email client know when I receive the modification (Step 3 above) to change the meeting I received and accepted in Steps 1 and 2?

I've been digging around the message headers for a common ID but nothing is jumping out. My suspicion is that it's using some hash of sender and date/time - but searching on-line isn't revealing the "how" - hence asking here :)

Answer 1

When a meeting invitation is sent, it includes a unique identifier known as the UID (Unique Identifier). This UID is generated by the calendar client of the person who created the meeting. When you accept the meeting, this UID is stored along with the other meeting details in your calendar.

When a meeting update is sent, it also includes the same UID. Your calendar client uses this UID to match the update to the existing meeting in your calendar. So, when you accept the updated meeting, your calendar client knows which meeting to update based on the UID.

This process is standardized across different calendar clients (like Google Calendar and Outlook) through a protocol called iCalendar, which is widely used for sending meeting invitations and updates over email.

So, it's not a hash of the sender and date/time, but a unique identifier that's used to track meeting invitations and updates.